Start-up system for forced flow vapor generator



Sept. 13, 1966 START-UP SYSTEM FOR FORCED FLOW VAPOR GENERATOR Filed 001). 22, 1964 M. WIENER 2 Sheets-Sheet l 14 29 s 1 1s 23 24 2s 22 19 55 6O 25 41 5O 6 63 44 3O 1 39 6 46\)Q&146A

INVENTOR. Murray Wiener BY ATTORNEY Sept. 13, 1966 M. w EN 3,271,961

START-UP SYSTEM FOR FORCED FLOW VAPOR GENERATOR Filed Oct. 22, 1964 2 Sheets-Sheet 2 41 m 20 25 so 6 66A 62 e3 43A 6 46A 39 4 4 46"\) INVENTOR.

Murray Wiener United States Patent 3,271,961 START-UP SYSTEM FOR FORCED FLOW VAPOR GENERATOR Murray Wiener, Akron, Ohio, assignor to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Oct. 22, 1964, Ser. No. 405,685 5 Claims. (Cl. 60105) This invention relates in general to a power plant system having a turbine arranged to be supplied with vapor from a forced circulation once-through vapor generator and more particularly to apparatus for and a method of starting up such a system.

The general object of the present invention is the provision of a starting system of the character described so constructed and arranged as to simplify starting procedme; to provide low cost, rapid, controlled start-ups; to provide adequate protection of the turbine and vapor superheating section of the vapor generator to the end that thermal stresses on this equipment are within acceptable limits at all times; to provide maximum heat recovery during starting up and low load operation; and to permit matching of the vapor temperature to turbine metal temperature on hot restarts and a minimum differential of vapor temperature and turbine metal temperature on cold starts.

More specifically, the invention is directed to improvements in the construction and operation of a start-up system of the type described in US. Patent No. 2,989,038 in which the discharge from the vapor generating section of a once-through boiler is passed into a flash tank, water from the flash tank is conducted to the inlet end of the vapor generating section of the boiler, vapor from the flash tank is passed through the vapor superheating section of the boiler and then condensed for return to the vapor generating section, and provisions are made for bypassing the flash tank when the working medium is properly conditioned for rolling and loading the turbine. A disadvantage of this system, as well as other prior startup systems, resides in its inability, especially during col-d start-ups, to provide low temperature steam to the turbine for rolling and loading. As a result, the turbine metal is subjected to excessive rates of temperature change. This drawback is eliminated in accordance with the present invention by special provisions for tempering vapor supplied to the turbine for rolling and loading.

The invention will now be described in further detail by reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a power plant system embodying the invention; and

FIG. 2 is a schematic diagram of a modified plant according to the invention.

As generators of the forced flow type are well known in the art, I have shown in the drawings the elements of such a generator in rudimentary form to assist in understanding my invention. While the starting system of the invention is particularly adapted for use in a forced flow vapor generating and superheating unit designed for the production of superheated vapor at pressures and temperatures below the critical pressure of 3206 psi. and the critical temperature of 705 F., a unit of this general construction being described and claimed in US. Patent No. 3,125,995, issued March 24, 1964, it will be understood that the invention starting system can also be advantageously used in a forced flow vapor generator designed for supercritical pressures and temperatures.

In the power plant system illustrated, feedwater is supplied by a boiler feed pump through a conduit 11 to a high-pressure heater 12, then passes through a conduit 13 to the circuitry of a vapor generator 14 of the once-through forced flow type having a series fluid flow 3 ,2 71,96 1 Patented Sept. 13, 1966 path including an economizer 16, vapor generating section 17, and superheater 18. Superheater 18 is connected for series flow of fluid from the vapor generating section by a conduit 19 and for series flow fluid to a highpressure turbine 22 by a conduit 23 containing a stop valve 24 and a control valve 26. Valve 24 is by-passed by a conduit 20 containing a stop valve 25. Fuel and air for combustion are admitted to the vapor generator 14 through conduits 27 and 28 respectively; and the gaseous products of combustion, after passing over the economizer and vapor generating and superheating surfaces, are discharged through an outlet, diagrammatically shown at 29. Valves 27A and 28A represent the customary regulating means for the fuel and air respectively.

During normal operation, the working medium passes successively through the economizer 16, vapor generating section 17 and superheater 18. The superheated vapor outflow of superheater 18 passes through conduit 23 to the high-pressure stage of vapor turbine 22 for expansion therein, with the vapor then exhausting through a conduit 30 to a main condenser 31, where it is condensed at a low pressure for return to the feedwater system. From the condenser the condensate passes by way of a conduit 32 to a pump 33 from which it discharges through a conduit 34 and low-pressure heater 36 to a direct contact type deaerating heater 37 which serves to boil the condensate to eliminate any entrained oxygen. Condensate from the deaerator passes through a conduit 38 to the suction side of feed pump 10 for return to the vapor generator.

In accordance with the invention, a special by-pass system is provided around the superheater and around the turbine. This system is constructed and arranged to obtain highest operational flexibility, to minimize heat losses, and to provide full thermal protection of superheater surface and the turbine; and is used for cold and hot start-ups, for low load operation and for shutdown or emergency trip of the vapor generator.

The superheater by-pass comprises a conduit 39 containing a pressure breakdown valve 41 and having one end connected and opening to conduit 19 and its opposite end connected and opening to a flash tank 42, which serves as a receiving vessel for the by-pass fluid and separates steam and water. In accordance with the invention, provisions are made for diverting flash tank steam to a position downstream of the turbine stop valve 24 by means of a conduit 55 containing a control valve 50 and a stop and check valve 60 and having its inlet end connected to flash tank 42 and its discharge end connected to conduit 23 at a point intermediate stop valve 24 and control valve 26. This particular feature will be hereinafter enlarged upon.

The turbine by-pass comprises a conduit 43 containing a pressure-reducing valve 44, spray attemperator 46 and attemperator control valve 46A, with conduit 43 having one end connected and opening to conduit 23 at a position intermediate superheater 18 and turbine stop valve 24 and its opposite end connected and opening to condenser 31. Spray attemperator 46 is provided to reduce steam temperature to conditions acceptable to the condenser.

Drainage from the flash tank is provided by a conduit 49 containing a deaerator water pegging valve 51 and extending between flash tank 42 and deareator 37; by a branch conduit 52 containing a valve 53 and having one end connected to conduit 49 at a point upstream of valve 51 and its opposite end connected to condenser 31; and by a branch conduit 65 containing a valve 70 and having one end connected to conduit 52 at a point upstream of valve 53 and its opposite end connected to the shell side of heater 12.

Steam connections from the flash tank to the turbine seals, deaerator and high-pressure heater provide steam for these components during start-up, thereby recovering heat for the cycle. Thus a conduit 54, controlled by a valve 56, extends between flash tank 42 and the shell side of heater 12; a conduit 57, having a valve 58, has one end connected to conduit 54 at a position upstream of valve 56 and its opposite end connected to deaerato-r 37; and a conduit 59, controlled by a valve 61 leads from the flash tank to the turbine seals. A steam line 62, containing a valve 63 and leading from flash tank 42 to conduit 43 at a position intermediate valve 44 and spray attemperator 46, acts as a flash tank over pressure control by allowing any excess steam to flow to condenser 31.

In a typical cold start-up of the power plant system illustrated, about one quarter full load water flow is established through the boiler feed pump which takes suction from deaerator 37 and forces fluid successively through conduit 11, heater 12, conduit 13, economizer 16, vapor generating section 17, and conduit 41 to flash tank 42. Tunbine by-pass valve 44, turbine stop valve 24, and turbine stop valve by-pass 25 are closed so that there is no flow through superheater 18. Valve 41 is set to maintain full load pressure at the outlet of the vapor generating section throughout the start-up.

From the flash tank 42 the water flows through conduit 49 to deaerator 37 and then back to feed pump 10. Initially valve 51 is set to maintain flash tank water level below normal. Firing is then commenced and gas temperature entering superheater 18 is held to approximately 1,000" F. As the enthalpy of the fluid entering the flash tank increases, more residual heat is introduced to the deaerator, causing deaerator and flash tank pressure to rise. When deaerator pressure reaches about 20 p.s.i.a., valve 51 starts to close and the flash tank level control valve 70' opens to allow flash tank drains to flow to the shell side of heater 12 to minimize heat loss, with the drains then passing through a conduit 71 to condenser 31. Conduit 71 is controlled by a valve 72. After 20 p.s.i.a. is reached in the deaerator, valve 51 controls deaerator pressure and the flash tank level control valve 70 now controls flash tank water level. If the flash tank water level exceeds the normal water level, valve 53 opens to divert the excess water to the condenser. Valve 70 is also controlled to close on high water level in heater 12.

As the temperature of the water entering the flash tank increases, flash steam becomes available and pressure in the flash tank continues to increase until the opening pressure of the seal steam control valve 61 is reached at which time separated steam is directed through conduit 59 and valve 61 to seal the turbine. When the turbine is sealed, condenser vacuum can be pulled. As the enthalpy of the fluid continues to rise, the flash tank pressure increases until the opening pressure of the deaerator steam pegging valve 58 is reached, at which point flash tank pressure is about 75 p.s.i.a. and steam from the flash tank goes to the deaerator 37 by Way of conduits 54 and 57 until the deaerator pressure reaches about 25 p.s.i.a. At this time valve 51 is closed, since it has a set point of 20 p.s.i.a. deaerator pressure, and valve 70 is open so that more of the flash tank water is diverted to the shell side of heater 12 and thence passes to condenser 31 by way of conduit 71. Thus feedwater temperature is increased and heat is recovered by the use of flash tank steam to deaerator 37 and flash tank drain water in the high-pressure heater 12. When the flash tank pressure is 50-80 p.s.i.a. above the pressure at which the deaerator is satisfied with the quantity of steam delivered, the high-pressure feedwater heater steam control valve 56 opens so that excess steam is diverted to the shell side of heater 12. As the flash tank pressure continues to rise, heater 12 pressure will rise, thus increasing feedwater temperature further and recovering more heat.

After deaerator and turbine seal requirements are satisfied, valve 44 is opened so that fluid is directed from the vapor generating section 17 through conduit 19 to and through superheater 18, and then through conduits 23 and 43 to condenser 31, thereby warming up such steam lines and the superheater. Valve 44 is throttled to maintain flow in the superheater and conduit 23 as required for warm up. When the temperature and flow rate of the steam in the turbine by-pass conduit 43 reaches predetermined conditions, then steam flow is diverted into the turbine by Way of valve 25 and valve 26.

Common practice in prior start-up systems for forced circulation boiler-turbine combinations has been to conduct flash tank steam to and through the superheater and thence through the turbine by-pass system to attain desired steam conditions before directing flow to the turbine. The prime concern during starting and loading of the turbine is the gradual and uniform heating of the turbine metal to assure that excessive thermal differences do not occur across any portion of the turbine metal. In this connection it is desirable to match the steam and metal temperatures as closely as possible to keep thermal stress at a minimum. One of the main drawbacks of most start-up systems of the character above described resides in their inability, particularly during cold start-ups, to provide low temperature steam to the turbine for rolling and loading, owing to the fact that the through flow of the superheater for turbine rolling is only 2 to 3 percent of full load steam flow. With such low steam flow, the temperature of the steam discharging from the superheater to the turbine approaches the flue gas temperature entering the superheater. Consequently, the turbine metal is subjected to excessive rates of temperature change. This condition is alleviated to some extent on forced circulation vapor generators having provisions for tempering the flue gases, but without such equipment the minimum start-up steam temperature obtainable with prior systems has been about 825850 F.

In accordance with the invention, flash tank separated steam is directed through conduit 55 for mixing with outflow of superheater 18 when the turbine is ready to be rolled. Combination stop and check valve 60 pre vents flow from the superheater back to the flash tank. The temperature of the separated steam leaving the flash tank is the saturation temperature corresponding to the pressure in the flash tank. Thus the start-up system of the invention makes it possible to temper the superheater outflow prior to and during rolling and loading of the turbine and to supply turbine rolling steam through a wide range of temperatures down to as low as about 400 F. While firing the boiler hard enough to make the required amount of steam for turbine rolling and loading.

More specifically, when the steam in conduit 43 is properly conditioned for rolling the turbine, steam flow to the turbine is controlled with turbine stop valve 25, allowing steam admission to the turbine with valve 26 wide open, which results in uniform heating of valve bowls, valve chests and turbine first stage shell area. Valve 25 is controlled to throttle the pressure of the steam down to a level suflicient to roll the turbine, this pressure being about 50 p.s.i.a. At the same time valve 24 is closed and valve 50 is controlled so that for each degree of opening of valve 25 there is a set opening of valve 50. Thus the turbine receives a mixture of superheated steam passing through conduit 23 and valve 25 and saturated steam passing through conduit 55 and valve 50. The proportions of superheated and saturated steam in the mixture passing to the turbine may be varied over a wide range by suitable adjustment of valves 50 and 25 to the extent that the turbine may be supplied with steam close to the saturation temperature corresponding to its pressure or steam superheated up to a temperature corresponding to the temperature of the steam discharging through valve 25.

Following the prescribed rolling period, at or near rated speed, the turbine is synchronized and loaded to 5 to 10 percent load by closing down turbine by-pass valve 44. Load may then be increased to about 20 percent of full load by increasing firing rate. At this time, flow to the flash tank will only constitute the difference between startup flow and that passing directly to the superheater and turbine via the valve 25.

At a predetermined point in the loading of the turbine, control of the turbine is transferred from valve 25 to valve 26. This is done by opening valve 25 while closing valve 26, producing a pressure increase between these valves. This pressure increase will tend to reduce the flow of flash tank steam through valve 50. When valve 25 is sufficiently open and valve 26 sufliciently closed so that the pressure upstream of valve 26 is higher than the flash tank pressure, valve 60 will close on check action. Now valve 24 can be opened wide and valve 25 closed, followed by the closing of valve 41. Thus flows to and from the flash tank cease and the pressure in the flash tank and by-pass system decays. Normal turbine extraction flows will replace flash tank steam and drain flows for deaeration and feedwater heating.

During the period after the flash tank reaches its normal operating pressure, any steam separated in the flash tank in excess of what is required for turbine sealing, deaeration, feedwater heating and turbine operation is discharged through the flash tank over pressure control valve 63 to the condenser. During a cold start, there is little or no steam flow to the condenser.

The operation sequence for hot restarts and intermediate starts is essentially the same as for a cold start, except that rolling steam for the turbine may be provided primarily through valve 25 if desired.

In the modification shown in FIG. 2, the turbine by-pass conduit 43A has its inlet end connected to conduit 23 at a position intermediate superheater 18 and valve 24 and its outlet end connected to superheater by-pass conduit 39 at a point downstream of valve 41. In addition, conduit 39 is provided with a spray attemperator 66 and attemperator control valve 66A at a location downstream of the point of intersection of conduits 39 and 43A. In operation of the system thus modified, deaerator and turbine seal requirements are satisfied in the same manner as described in the FIG. 1 embodiment. Then valve 44 is opened so that fluid is directed from the vapor generating section 17 through conduit 19 to and through superheater 18 for warming up such steam lines and the superheater, and then through conduit 43A to conduit 39 for flow to flash tank 42 along with the fluid passing directly from the vapor generating section 17 to conduit 39. Valve 44 is throttled to maintain flow in the superheater and conduit 23 as required for warm-up and to match the pressure on the downstream side of valve 41.

When the steam in conduit 43A is properly conditioned for rolling the turbine, steam flow to the turbine is controlled in the same manner as set forth in the FIG. 1 embodiment, With valves 25 and 50 being regulated so that the turbine steam supply is at the desired temperature and pressure. As in the FIG. 1 system, upon reaching a predetermined point in the loading of the turbine, control of the turbine is transferred to valve 26, followed by closing of valve 60 on check action, opening of valve 24, and closing of valve 41.

While in accordance with the provisions of the statutes, I have illustrated and described herein the best form and mode of operation of the invention now known to me, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by my claims, and that certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. The method of starting a power plant system in which, during normal operation, vaporization fluid is successively passed through a heating zone and a superheating zone to a vapor turbine, said method comprising passing a vaporizable fluid under substantial pressure through the heating zone in indirect heat transfer relation with heating gases, passing a portion of the heating zone fluid outflow to a separating zone While reducing its pressure to cause part of the fluid to flash into vapor, passing the remaining portion of the heating zone fluid outflow to and through the superheating zone in indirect heat transfer relation with the heating gases, effecting separation of the vapor portion of the fluid in the separating zone, passing separated vapor from the separating zone to a position downstream of the superheating zone for mixing with superheating zone vapor outflow, passing the vapors so mixed to the turbine for rolling and loading thereof.

2. The method of starting a power plant system in which, during normal operation, vaporizable fluid is successively passed through a heating zone and a superheating zone to a vapor turbine, said method comprising passing a vaporizable fluid under substantial pressure through the heating zone in indirect heat transfer relation with heating gases, passing a portion of the heating zone fluid outflow to a separating zone while reducing its pressure to cause part of the fluid to flash into vapor, passing the remaining portion of the heating zone fluid outflow through the superheating zone in indirect heat transfer relation with the heating gases, condensing the superheating zone outflow and recirculating it to the heating zone until predetermined conditions of fluid temperature and pressure for starting the turbine are attained at the outlet of the superheating zone, effecting separation of the vapor portion of the fluid in the separating zone and passing at least a portion of the separated vapor to a position downstream of the superheating zone for mixing with vapor discharging from the superheating zone, and passing the vapors so mixed to the turbine for rolling and loading thereof.

3. The method of starting a power plant system in which, during normal operation, vaporizable fluid is successively passed through a heating zone and -a superheating zone to a vapor turbine, said method comprising passing a vaporizable fluid under substantial pressure through the heating zone in indirect heat transfer relation with heating gases, passing a portion of the heating zone fluid outflow to a separating zone while reducing its pressure to cause part of the fluid to flash into vapor, effecting separation of the vapor portion of the fluid in the separating zone and then condensing the separated vapor and recirculating the condensate to the heating zone, recirculating separated liquid from the separating zone tothe heating zone, passing the remaining portion of the heating zone fluid outflow through the superheating zone in indirect heat transfer relation With the heating gases, condensing the superheating zone outflow and recirculating it to the heating zone, recirculating fluid from the superheating and separating zones to the heating zone until predetermined conditions of fluid temperature and pressure for starting the turbine are attained downstream of the superheating zone, passing a portion of the superheating zone outflow to the tunbine for rolling and loading thereof while passing a portion of the separating zone vapor outflow to a position downstream of the superheating zone for mixing with the vapor flowing to the turbine, discontinuing the by-passing of fluid from the heating zone to the separating zone and from the superheating zone to the heating zone so that all the fluid entering the fluid heating zone passes successively through the heating zone and superheating zone to the turbine.

4. In a power plant having a turbine, a forced flow vapor generator having a through-flow circuit including a fluid heating section and a superheater section connected for series flow of fluid from the fluid heating section and to the turbine, and a pump supplying vaporizable fluid to the fluid heating section, means for starting-up the vapor generator and turbine comprising a cut-ofi valve arranged to control fluid flow between the superheating section and the turbine, a flash tank, means for passing a portion of the fluid heating section outflow to the flash tank while passing the remainder of the fluid heating section outflow to the superheater section, said last named means including a conduit connecting the fluid heating section to the flash tank and arranged to convey a portion of the discharge from the fluid heating section to the flash tank when said cut-off valve is closed, and a pressure reducing valve in said conduit arranged to cause fluid discharge from the fluid heating section to flash into vapor, means for passing separated liquid from the flash tank to the fluid heating section, means for passing superheater outflow to the fluid heating section, and means for passing separated vapor from the flash tank to the downstream side of said cut-ofl valve for mixing with super-heater outflow when said cut-off valve is open.

5. In a power plant having a turbine, a forced flow vapor generator having a through-flow circuit including a fluid heating section and a superheater section connected for series flow of fluid from the fluid heating section and to the turbine, and a pump supplying vaporizable fluid to the fluid heating section, means for startingup the vapor generator and turbine comprising a cut-01f valve arranged to control fluid flow between the superheating section and the turbine, a flash tank, means for passing a portion of the fluid heating section outflow to the flash tank while passing the remainder of the fluid heating section outflow to the superheater section, said last named means including a conduit connecting the fluid heating section to the flash tank and arranged to convey a portion of the discharge from the fluid heating section to the flash tank when said cut-01f valve is closed, and a pressure reducing valve in said conduit arranged to cause fluid discharge from the fluid heating section to flash into vapor, means for passing separated liquid from the flash tank to the fluid heating section, means for passing super-heater outflow to said conduit at a position downstream of said pressure reducing valve, and means for passing separated vapor from the flash tank to the downstream side of said cut-off valve for mixing with superhe-ater outflow when said cut-ofl valve is open.

' References Cited by the Examiner UNITED STATES PATENTS 3,175,367 3/1965 Gorzegno et a1. 122-406 X 3,183,896 5/1965 Lytle et al 122406 3,220,193 11/ 196-5 Strohmeyer -105 X CHARLES .i. MYHRE, Primary Examiner. 

1. THE METHOD OF STARTING A POWER PLANT SYSTEM IN WHICH, DURING NORMAL OPERATION, VAPORIZATION FLUID IS SUCCESSIVELY PASSED THROUGH A HEATING ZONE AND A SUPERHEATING ZONE TO A VAPOR TURBINE, SAID METHOD COMPRISING PASSING A VAPORIZABLE FLUID UNDER SUBSTANTIAL PRESSURE THROUGH THE HEATING ZONE IN INDIRECT HEAT TRANSFER RELATION WITH HEATING GASES, PASSING A PORTION OF THE HEATING ZONE FLUID OUTFLOW TO A SEPARATING ZONE WHILE REDUCING ITS PRESSURE TO CAUSE PART OF THE FLUID TO FLASH INTO VAPOR, PASSING THE REMAINING PORTION OF THE HEATING ZONE FLUID OUTFLOW TO AND THROUGH THE SUPERHEATING ZONE IN INDIRECT HEAT TRANSFER RELATION WITH THE HEATING GASES, EFFECTING SEPARATION OF THE VAPOR PORTION OF THE FLUID IN THE SEPARATING ZONE, PASSING SEPARATED VAPOR FROM THE SEPARATING ZONE TO A POSITION DOWNSTREAM OF THE SUPERHEATING ZONE FOR MIXING WITH SUPERHEATING ZONE VAPOR OUTFLOW, PASSING THE VAPORS SO MIXED TO THE TURBINE FOR ROLLING AND LOADING THEREOF. 